This invention relates to a method for the vacuum depositing of corrosion and oxidation resistant alloy coatings on all sides of curved turbine buckets having leading edges and trailing edges, with the simultaneous rotation of the turbine buckets about their longitudinal axis in the vapor stream above a vapor source.
In connection with turbine buckets, there is a distinction to be made between runner buckets and nozzle buckets. The runner buckets consist generally of a blade and a base, the base having on the side facing the blade a substantially rectangular surface that is virtually perpendicular to the blade. In the case of the nozzle buckets, as a general rule the end farthest from the base is provided with an enlargement analogous to the base, the surfaces of the bases and of the enlargements opposite them making up a cylindrical ring after all of the buckets have been installed. The "longitudinal axis" of such buckets is to be interpreted as an imaginary axis running parallel or substantially parallel to the generatrices of the bucket blade. The longitudinal axis can coincide with the axis of rotation of the substrate holder during the rotation of the bucket in the vapor stream, but it can also be at a distance therefrom, i.e., the bucket rotates about its longitudinal axis while the longitudinal axis rotates about the axis of rotation of the substrate holder.
It is known to cause both axially symmetrical bodies as well as bodies of complex shape to rotate about one or more axes in the vapor stream for the purpose of coating the substrates on all sides with a condensed layer of material. Particularly in the case of complex composite rotatory movements, as for example in the case of the known involute movements, reliance is placed on laws of probability whereby the entire surface that is exposed to the vapor will be coated with the evaporated material with at least some degree of uniformity. The relative uniformity of the deposit on the substrates is favored by the diffuse character of the vapor stream and to some extent by the collision of vapor particles with molecules of the residual gas atmosphere. Reliance on probability, however, is no longer adequate for the coating of precision parts such as turbine buckets. Especially in the case of gas turbines, a high temperature is desired at the output of the combustion chamber and at the input of the turbine stages for the purpose of improving efficiency and thereby reducing the specific fuel consumption. This heating has resulted in a differentiated make-up especially of the runner buckets. The core of these buckets, which is the substrate during the vacuum coating, consists as a rule of highly heat resistant steel alloys of known composition. These alloys, however, are not sufficiently corrosion resistant and oxidation resistant in the desired temperature range. The manufacture of the whole turbine bucket from appropriately resistant material is unfeasible for reasons of strength. Consequently, manufacturers have resorted to applying corrosion resistant and oxidation resistant surface coatings to turbine buckets for the purpose of increasing their life and safety factor. It must be remembered that the lives of hundreds of human beings depend to no small degree on the quality of turbine buckets. The problems involved and methods for coating gas turbine buckets with appropriate surface materials are described in a publication by the Chromalloy American Corporation, N.Y., entitled, "High Temperature Resistant Coating for Super-Alloy" by Seelig et al. However, the publication does not say in what manner a uniformly thick, dense, adherent coating, free of cracks and voids, can be obtained on the entire surface of buckets.